To investigate the novel function of AIP1 in VEGFR-3 signaling, and VEGFR-3-dependent angiogenesis and lymphangiogenesis.

AIP1, a signaling scaffold protein, is highly expressed in the vascular endothelium. We have previously reported that AIP1 functions as an endogenous inhibitor in pathological angiogenesis by blocking VEGFR-2 activity. Surprisingly, here we observe that mice with a global deletion of AIP1 (AIP1-KO) exhibit reduced retinal angiogenesis with less sprouting and fewer branches. Vascular endothelial cell (but not neuronal)-specific deletion of AIP1 causes similar defects in retinal angiogenesis. The reduced retinal angiogenesis correlates with reduced expression in VEGFR-3 despite increased VEGFR-2 levels in AIP1-KO retinas. Consistent with the reduced expression of VEGFR-3, AIP1-KO mice show delayed developmental lymphangiogenesis in neonatal skin and mesentery, and mount weaker VEGF-C-induced cornea lymphangiogenesis. In vitro, human lymphatic EC with AIP1 siRNA knockdown, retinal EC and lymphatic EC isolated from AIP1-KO all show attenuated VEGF-C-induced VEGFR-3 signaling. Mechanistically, we demonstrate that AIP1 via vegfr-3-specific miR-1236 increases VEGFR-3 protein expression, and by directly binding to VEGFR-3 enhances VEGFR-3 endocytosis and stability.

Our in vivo and in vitro results provide the first insight into the mechanism by which AIP1 mediates VEGFR-3-dependent angiogenic and lymphangiogenic signaling.